1. Field of the Invention
The invention relates to an apparatus and various methods used therein for continuous non-invasive measurement of the pressure of a pulsatile fluid flowing through a flexible tube over relatively long time periods, with particular applicability to the measurement of human arterial blood pressure and other related cardiovascular parameters.
2. Description of the Prior Art
Often a need arises to monitor pulsatile fluid pressure in a vessel where a number of practical considerations preclude direct invasive measurement, i.e. using an appropriate pressure sensor directly implanted through the vessel and maintained in a suitable position within the fluidic flow. Considerations of this nature include avoiding: contamination of the fluid or its immediate environment by any foreign matter carried by the sensor, coagulation of the fluid, corrosive deterioration caused to the sensor by direct contact with the fluid, fluid loss from the vessel, or physical damage to the flexible vessel that contains the fluid.
These considerations have particular applicability to the measurement of arterial blood pressure of human (or other animal) patients or subjects. In practice, invasive pressure monitoring generally entails a surgical cut-down and arterial penetration using a hypodermic needle (cannula) through which pulsatile fluidic forces attributable to arterial blood flow are routed to a suitable pressure transducer. However, various medical health care risks associated with invading the human body, such as clotting, infection, emboli obstructions to flow, and/or major blood loss generally limit the use of invasive blood pressure monitoring systems to the most critical of acute care hospital patient monitoring situations. To minimize these risks, invasive monitoring is almost always used in conjunction with intravenous application of fluids to the patient. Disadvantageously, the various fluidic, mechanical and electrical components generally used in invasive systems are not only complex and fragile, but also require specialized calibration before use, as well as frequent surveillance by specially qualified staff. In spite of this surveillance, invasive systems often do not remain calibrated during prolonged periods of use and, as a result, often produce inaccurate and potentially misleading patient blood pressure measurements.
Consequently, over the years several techniques have been developed for non-invasive arterial blood pressure measurement. In general, these techniques rely upon attaching an inflatable cuff around an extremity (limb), which is typically an upper arm, of a human patient. Once attached, the air pressure existing within the cuff is increased to a value commonly referred to as "suprasystolic," i.e., a pressure in excess of that minimally necessary to completely occlude a major artery existing within the extremity and situated near its surface. Thereafter, blood pressure is most commonly estimated by detecting well-known "Korotkoff" sounds using a stethoscope, a microphone, or an ultrasonic detector placed on the limb near the artery. These Korotkoff sounds are produced by the artery and, more particularly, by disturbances in the arterial blood flow due to partial occlusions of the artery caused by the externally applied cuff pressure. As the cuff pressure decreases and the extent of occlusion is reduced, various classic phases of sound change are usually heard until the artery is no longer occluded by any appreciable amount. Specifically, as the cuff pressure is reduced from suprasystolic, the maximum value of pulsatile blood pressure commonly referred to as systolic pressure, is usually taken to be equal to the cuff pressure at the time the first Korotkoff sound is detected. Thereafter, the minimum or so-called "diastolic pressure" value of pulsatile arterial pressure is usually identified in conjunction with the occurrence of one of two other Korotkoff phases: either the so-called fifth phase when silence occurs or the so-called fourth phase which corresponds to a cuff pressure of about 5-10 mm(Hg) higher than that occurring at the fifth phase. Manual pressure readings for systolic and diastolic are determined by identifying each desired phase, and, as the cuff pressure continually decreases, simultaneously noting the scale value in mm(Hg) that corresponds to the height of a mercury column (or the pointer on an aneroid gauge) which is pneumatically connected to the cuff air pressure. Devices of this sort are commonly referred to as "sphygmomanometers."
Unfortunately, the accuracy of any non-invasive sphygmomanometer type blood pressure measurement system, typified by that described above, is largely dependent on the skill and hearing acuity of its user in detecting the rather subtle sound changes (such as the very gradual transition to silence after the fourth phase occurs), and, simultaneously therewith, determining the exact level of the mercury column. In addition, dexterity, sensory limitations and inexperience of the user; interference of environmental noises, and the need to frequently calibrate the measurement system often occur and all contribute to produce highly inconsistent results. This inconsistency is a widely known characteristic of sphygmomanometric systems.
Consequently, in an endeavour to minimize inconsistent results, many attempts have occurred in the art to automate the process of sphygmomanometric measurement. Specifically, these attempts involve using electronic processing circuitry to automatically determine the desired phases of Korotkoff sound change and the simultaneously occurring systolic and diastolic cuff pressure values.
These attempts are typified by the systems disclosed in U.S. Pat. Nos. 3,581,734 (issued to Croslin et al on June 1, 1971); 4,245,648 (issued to Trimmer et al on Jan. 20, 1981) and 4,271,844 (issued to Croslin on June 9, 1981). Each of these three patents discloses a computerized sphygmomanometer measurement system in which an occlusive cuff is attached around a limb of a patient. The cuff is then inflated, either manually or automatically by an electrically driven air pump which is controlled through either a computer or a hard-wired digital circuit. In each of these systems, the cuff is inflated to a suprasystolic occluding pressure prior to taking (sampling) any blood pressure measurement data. Then, by automatically undertaking various detection and determination processes, as well as deflating (bleeding-down) the occlusive cuff, these systems attempt to eliminate many of the above-described manual steps that can cause measurement error in manual sphygmomanometer systems known to the art. Unfortunately, these automated sphygmomanometer systems are incapable of reliably and consistently representing the true status of blood pressure, in the same manner as provided by direct invasive monitors that are widely accepted as the "standard of blood pressure measurement accuracy".
Specifically, sphygmomanometer systems commonly produce misrepresentative results due to a number of factors that are generally transparent to or incapable of being compensated by the practioner-user. One such factor is the impracticality of causing the pressure of any deflating occlusive cuff known in the art to be made equal to the true peak systolic pressure value of any one or more intra-arterial pressure waveforms such that the measured cuff pressure is an accurate representation of systolic. This impracticality results from the fact that any pulsatile intra-arterial peak pressure value exists for only a short interval of time, (e.g. usually less than 5% of the time). Inasmuch as the timing of cuff pressure bleed-down is a random variable, the cuff pressure is typically lower than true systolic peak pressure by random amounts, e.g., up to 10 mm(Hg), depending on the deflation rate used before the desired Korotkoff or pressure displacement waveform signal occurs--which indicates when the cuff pressure is to be measured and designated as the systolic measurement value. A second factor is the apparent lack of any uniform and accurate diastolic determination method in systems known to the art. Specifically, either one of two Korotkoff phases, i.e., the fourth and fifth phase, each of which produces consistently different measurement values have found wide use in prior art systems. Also, these diastolic measurement methods known in the art often determine diastolic pressure as the value of occlusive cuff pressure whenever it exceeds a certain threshold. Such a threshold value is primarily dependent on the cuff pressure at the mean arterial blood pressure instead of other more relevant and accurate independent physiologic variables. Moreover, these methods, are generally premised on an assumed linear relationship existing between amplitude values at mean and diastolic pressure and linearly extrapolate the diastolic pressure value based upon the mean pressure value. Accordingly, these measurement methods have the unfortunate effect of assuming somewhat erroneously, that a single fixed linear elasticity relationship defines the stress/strain (e.g. pressure/displacement) characteristics of the artery walls of all patients for whom blood pressure is to be non-invasively measured--thereby resulting in an inaccurate determination of the true diastolic pressure value. Lastly, a third factor is that systolic and diastolic pressures commonly vary by differing amounts from one heart-beat to the next due to several physiologic factors for both normal and critically ill patients. Unfortunately, any combination of these factors serves to over- or under-state not only the value of blood pressure, but also more importantly changes in arterial blood pressure occurring over time between successive measurements taken from any one patient.
Moreover, these prior art systems not only lack the capability of accurately portraying the arterial blood pressure associated with individual heart-beats, but also disadvantageously they generally produce only one systolic and diastolic reading during a measurement cycle that can span between 20 and 100 successive heartbeats. To properly represent the true status of blood pressure on a heart-beat to heart-beat basis, a much higher sampling rate is necessary. However, if any of these sphygmomanometer systems were used to measure arterial blood pressure variations on a continuous heartbeat-by-heartbeat basis, then the occlusive cuff would need to be repetitively and successively inflated to a suprasystolic pressure and possibly to atmospheric pressure over many short successive intervals of time, such as, for example, 10 times per second, as is disclosed in U.S. Pat. No. 4,343,314 (issued to Sramek on Aug. 10, 1982). Unfortunately, prevailing medical opinion is that any patient wearing an occlusive cuff cannot be continuously subjected to either elevated cuff pressures more than about 30% of the time during which the cuff is being worn or repetitive cycling of cuff pressure between suprasystolic and sub-diastolic pressures on the order of more than once every one to three minutes, without experiencing significant discomfort, trauma, and possible physiologic damage.
A typical repetitive cycling sphygmomanometer measurement system known to the art which attempts to minimize patient discomfort is disclosed in U.S. Pat. No. 4,378,807 (issued to Peterson et al on Apr. 5, 1983). As described therein, a control circuit automatically initiates one cycle of occlusive cuff inflation and bleed-down deflation only after a rather long pre-defined interval of time typically on the order of 7.5 to 60 minutes has elapsed. Unfortunately, pressure readings (one systolic and one diastolic) are only taken at the conclusion of this relatively long interval. As a result, the amount of measurement data is insufficient to determine short and long term trends and variability in blood pressure, as well as to identify, with any degree of reliability, any irregular heart-beat pulsations. Thus, such a system is unsuitable for prolonged continuous blood pressure monitoring of the critical patients. Consequently, invasive pressure monitoring systems--even in spite of their attendant health risks, as discussed above--are used to continuously measure and display the pressure waveform for each heart-beat and to compute the systolic, diastolic and mean pressure values based upon averages of a number (typically 4-6) of successive heartbeats.
An alternate well-known scheme of non-invasive monitoring involves measuring arterial wall displacement (i.e., radial distension of the artery wall) produced by pulsatile arterial blood pressure and then translating the measured displacement into an instantaneous blood pressure value. These measurements and translations would, if performed at a sufficiently rapid rate, appear to be continuous, i.e. result in the display of an uninterrupted trend of sequentially-occurring pressure waveforms showing substantial detail of the pulsatile nature of each waveform, much in the same fashion as obtained through an invasive monitor. See, for example, P. Flaud et al, "Pulsed Flows in Viscoelastic Pipes. Application to Blood Circulation", Journal of Physics (France) Vol 35, No. 11, Nov. 1974, pages 869-882 and P. Flaud et al, "An Experimental Device for Modelling Arterial Blood Flow," Review of Physical Applications (France) Vol. 10, No. 2, March 1975, pages 61-67, which disclose that radial displacement of the arterial wall is related to intra-arterial blood pressure changes. However, the relative magnitude of wall displacement is also directly related to the elasticity of the wall of the arterial vessel. Unfortunately, arterial elasticity not only varies significantly from patient to patient but also varies at different locations along each artery, as well as at different times for the same patient. Thus, a noninvasive pressure monitoring system that relies on relative arterial wall displacement, requires that its measurements first be calibrated against pressure measurements taken by a separate reference device, such as an occlusive cuff, which would then serve as a calibration reference for subsequent pressure values based upon arterial wall displacement measurements.
Hence, in view of the drawbacks associated with prior art non-invasive measurement systems, continuous blood pressure monitoring systems known and used in the art are generally invasive and thus rely on intruding a major artery of the patient.